To produce castings by gravity pouring (hereinafter also called “pouring”), a gas-permeable casting mold composed of sand particles (so-called sand mold) is most generally used. With such a gas-permeable casting mold, a gas (generally air) remaining in a cavity of a particular shape is pushed out of the cavity by a metal melt (hereinafter also called “melt”), and the melt is formed into a casting having substantially the same shape as the cavity. The cavity of the casting mold generally includes a sprue, a runner, a feeder and a product-forming cavity, into which a melt is supplied in this order. In the conventional technologies, when a melt head in the sprue becomes high enough to fill a product-forming cavity, the pouring of the melt is finished.
A solidified melt forms a casting integrally extending from the sprue to the runner, the feeder and the product-forming cavity. The feeder is not an unnecessary portion for obtaining sound castings, while the sprue and the runner are merely paths for a melt to reach the product-forming cavity, which need not be filled with the melt. Thus, as long as a melt filling the sprue and the runner is solidified, drastic improvement in a pouring yield cannot be expected. In the case of castings integrally having unnecessary portions, considerable numbers of steps are needed to separate cast products from unnecessary portions, resulting in low production efficiency. Accordingly, the sprue and the runner pose large problems in increasing efficiency in gravity casting.
Recently, a revolutionary method for solving the above problems has been proposed by JP 2007-75862 A and JP 2010-269345 A. To fill a desired cavity portion, part of a cavity in a gas-permeable casting mold, this method pours a metal melt into the cavity by gravity in a volume smaller than that of an entire casting mold cavity and substantially equal to that of the desired cavity portion; supplies a compressed gas to the cavity through a sprue before the melt fills the desired cavity portion; and then solidifies the melt filling the desired cavity portion. By this method, it is expected to make it substantially unnecessary to fill a sprue and a runner with a melt, because pressure to be obtained by the melt head height is given by the compressed gas.